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April 2003

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From:
"Miller, Greg @ DISPLAYS" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Wed, 16 Apr 2003 09:12:15 -0400
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Peter, the probe should be temperature compensated so that it reads the same
resistivity of an extract solution regardless of the temperature.  During
normal use, there are sources of heat like power supplies, pump motors,
...etc. that increase the extract solution temperature.  Changes in
temperature will change the resistivity value, so you need to eliminate that
source of error.  This change is also a function of alcohol concentration,
so you will need to control your IPA concentration within +/- 1% throughout.
I know the Ionographs have some temperature compensation in the software to
address this (having worked with Alpha for 10 years), but I cannot speak for
other instrument suppliers.

As for guage R & R, you need to document how repeatable your measurements
are.  Use #3 standardizing solution, and test while plotting known
quantities of NaCL theoretical vs amount actually measured.  Use a precision
pipette such as an eppendorf or equivalent, which is commonly used in the
medical industry for being able to dispense precise amounts.

One final note.....in the high resistivity ranges (400 meg ohm plus), the
extract solution will be more stable by maintaining an alcohol concentration
of 75 -78% vs 72 - 74%.  Yes you are loosing some of the benefit of water.
But, from a users perspective, your tests will be more repeatable.   Good
Luck

Greg Miller
L-3 Communications Displays
Alpharetta, GA




-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Tuesday, April 15, 2003 9:01 PM
To: Miller, Greg @ DISPLAYS
Cc: TechNet E-Mail Forum.
Subject: RE: [TN] ICT machines and high cleanliness requirements



Brilliant, Greg,

Just the sort of answer I'm looking for. I'll need to check to see if the
resistivity options on the Zero Ion machine we have to play with will allow
me to choose my own level, or if I'm stuck with the 60MOhms or 150MOhms it
offers as standard.

Can you help tell me how I would temperature-compensate the probe and why
this is important? It sounds like you have to fool the machine into
believing something that isn't, so I'ld like to understand the theory
behind the adjustment. Also "guage R&R"? Need some help on what this
reference means, as I haven't met it yet.

I've seen for myself the amazingly rapid influence that atmospheric Carbon
Dioxide has on the test solution, even at 150MOhm, so I can well imagine
what it would do at 450MOhms+. We'll take precautions against it when we
need to (i.e. to combat too many indications of boards not passing the
test).

Peter.



"Miller, Greg @ DISPLAYS" <greg.miller    15/04/2003 08:33 PM


              To:  "TechNet E-Mail Forum." <[log in to unmask]>, DUNCAN
Peter/Asst Prin Engr/ST
              Aero/ST Group@ST Domain

              cc:

              Subject: RE: [TN] ICT machines and high cleanliness
requirements











Having worked with both static and dynamic ionic contamination testers for
a
number of years in many different applications, the only way you can
measure
less than 1 micro gram equivalent (total) is to use a low volume (1 gallon
max), dynamic tester with a starting cleanliness level of at least 450 meg
ohm.  The resistivity probe must be temperature compensated from 22 to 38
C.


My experience in using the Ionograph 500 M SP shows that a level of 0.2
micrograms total can be measured, but you need to perform your own studies
using a precision pipette. Guage R & R is a must at these levels.
Unfortunately, when an alcohol/water solution is 400+ meg ohms or cleaner,
carbon dioxide will influence the reading.   As CO2 dissolves, derivatives
of carbonic acid form and ionize in solution.  If you want to see for your
self, open the lid and gently blow (quick puff)  on the solution and watch
the resistivity drop.  Perhaps testing under nitrogen will help.

Greg Miller



-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: Tuesday, April 15, 2003 5:39 AM
To: [log in to unmask]
Subject: Re: [TN] ICT machines and high cleanliness requirements


Brian,

Very many thanks for replying to my post. Part of your answer begs another
question - if conductivity is linear and can accurately reflect
contamination levels down to virtually zero, why are all the Ionic
Cleanliness Test machines designed to measure resistivity and not
conductivity?

I don't know how to set about coverting a machine to read conductivity of a
liquid instead of resistivity. Is there an easy way to do it, or has no-one
tried? I wouldn't want to just use a formula to convert the resistivity
reading into a conductivity value, since I don't trust the resistivity
readings at low contamination levels in the first place. There would be no
point, as the results wouldn't be so linear.

Sorry, I'm going to cross-examine further - using a 50:50 water:alcohol mix
will certainly help dissolve more of the non-water-soluble residues and
thereby release more ionic contamination into the test solution to be
measured. That would prove that you have a dirtier board than you
previously thought, and prove that cleanliness measurement is a relative
thing, not an absolute. It does not, however, improve the linearity or
correlation of the readings the machine gives against the amount of
contamination in the test solution - the higher amount of contamination
might just take the readings into a region of the correlation curve where
they coincide better, but that's all.

I agree with your other comments about minimising atmospheric CO2
absorbtion and restricting test-run times to 15 mins max (...?), although I
was finding that  at least one of the machines was reading low at the low
contamination level end of my testing - bad enough if there had been no CO2
influences on the results, almost worse if there were such influences,
since the departure from accurate figures would have been even worse.

I should have the courage of my convictions - it had penetrated me
reasoning that static technology ought to be better for measuring low
contamination levels than dynamic. However, we have a dynamic machine in
our hands for playing with, and a good price is being asked if we want to
buy it. The man holding the budget is keen, even if he doesn't seem to
understand the subtle minutiae of the measurement process and its pitfalls.
We're 99% of the way to clinching the deal already, but I'm still not
convinced that it can measure that well without a lot of work to correlate
the readings with actual contamination levels - i.e. so an operator can
look up the machine reading on a chart and get a "true" contamination
figure.

I would still like to know, though, if there is work being done to produce
machines that are 'comfortable' with measuring low contamination levels to
replace those that are operating right on their limits (or beyond) to
measure low levels. Saying a machine is quite capable of reading 0.2ug NaCl
eq-cm^-2, without taking the board area into account makes such a claim
meaningless. Many a machine will return such a cleanliness level reading if
you have a large enough board area to extract lots of contamination from.
The real question is "What is the smallest gross weight of contamination
that a machine can accurately measure, before it is divided by the board
area?" If it can measure accurately all the way to zero ug (either by
measuring resistivity or by measuring conductivity, that's the machine for
me! Anybody have one at a reasonable price?

Thanks again.

Peter



Brian Ellis <[log in to unmask]>      15/04/2003 03:20 PM

              To:  "TechNet E-Mail Forum." <[log in to unmask]>, DUNCAN
Peter/Asst Prin Engr/ST
              Aero/ST Group@ST Domain
              cc:
              Subject: Re: [TN] ICT machines and high cleanliness
requirements








Peter

As the original designer of the Contaminometer, but no longer involved
with it, in any way, I guess that I'm as qualified as any to respond to
you.

You are partially right, but not entirely. First of all, it must be said
that the 1.56 ug/cm2 eq. NaCl or, better, ug.cm^-2 eq. NaCl(which is the
correct way of expressing the units cf. ISO 2000) dates from an
interpretation of Hobson's original tests, way back in 1969. Please
don't tell me that modern electronics are the same as they were 34 years
ago. This figure is as antiquated as the dinosaur and has no meaning
today. The voltage gradients on our substrates have increased by at
least an order of magnitude, actually to the limit of acceptable with
HDIS, even for low voltage operation. I therefore concur that 0.2 ug/cm2
eq. NaCl would seem reasonable for modern electronics with conductor
spacings of less than 0.2 mm, **provided** that it is measured
reasonably accurately and for hi-rel end-products.

So, how can this be done? First of all, you need to maximise the
sensitivity. The first and obvious way is to use the UK Mod standard
solution of 50% v/v IPA and not the US MIL standard of 75%, which is a
relic of Hobson's work which had an integration time of 1 min, because
it was sprayed on from a wash bottle, and not immersed with longer
integration times. The 50% solution gives twice the sensitivity of the
75% solution.

Secondly, contamination in these equipments is directly and linearly
proportional to the delta conductivity, so is inversely and non-linearly
proportional to the delta resistivity. Unfortunately, for fine
measurements, because resistivity meters have a range, you are working
at the cramped end of the scale. It is therefore more accurate to use
conductivity with its linear scale.

Then the board size enters into play. If you are stuck with a small
board and a large tank, you simply measure on a plurality of boards. For
example, if you have a tank size of, e.g., 250 x 350 mm, you can put in
8 boards 100 x 100 mm at a time, to obtain maximum sensitivity. Of
course, your answer will be an average of the eight, but this is not a
disadvantage.

Then there is the reduction of errors. These can come from many sources.
A bad one comes from CO2 absorption. This can be reduced by presenting a
minimal surface area of solution at the air interface (rectangular,
rather than funnel-shaped, tanks) and further reduced by software
compensation of the absorption, as well as keeping the lid on during the
test, with a minimal air volume over the solution. Another point is to
keep the test down to under, say, 15 minutes, so that what you measure
is the dissolved contamination and not leachates from the substrate or
the instrument constructional materials. For the same reason, you should
use solution at low temperatures (20-25 deg C). At higher temperatures,
the heterogeneous structure of the polymeric surfaces tend to open and
provoke more leaching from the substrate.

I won't enter into the so-called "static" and "dynamic" methods. Our old
CM-5 and MCM-2 models could use either. Each had advantages and
disadvantages but, overall, the "static" method is the better,
especially for low contamination levels. (I put the words between
inverted commas, "", because they are very misleading misnomers.)

If you observe all these points, it is possible to have a reasonable
accuracy at 0.2 ug/cm2 eq. NaCl.

For the anecdote, I used to manufacture the Microcontaminometer MCM-1
and MCM-2, which had a sensitivity that it could measure down to better
than 0.05 ug eq. NaCl absolute. This would have measured better than
0.01 ug/cm2 eq. NaCl on a 100 x 100 mm board (it had an interchangeable
tank for a 4" hybrid substrate). I believe Multicore stopped the MCM-2
in 1993. It was partially replaced by the CM-20, but I don't know what
happened to that. I can only answer for the CM-1 to CM-5 and MCM-1 to
MCM-2 series, which were the ones I designed.

Anyway, what you wish to do is possible, provided you have the right
instrument and know how to use it correctly.

Brian

[log in to unmask] wrote:
> Morning, All
>
> I was interested to see the cleanliness level of <0.2ug NaCl eq/cm^2 that
> is being adopted by many manufacturers, actually being stated in a data
> booklet published by Concoat Systems. Having done a bit of poking around
> into the world of cleanliness testing, I have a question for discussion
if
> you're up for it:
>
> Most ionic cleanliness testers are still marketed as measuring
cleanliness
> to the level specified in the MIL and IPC specs (1.56ug NaCl eq/cm^2).
Fair
> enough. Having looked quite closely into such machines as the Ionograph,
> Concoat's own CM11 Contaminometer, Omegameters and the Zero Ion, which
> seems to be teacher's pet in the IPC books (TR-583, anyway). I find that
> none of the machines reads at all reliably at levels as clean as 0.2ug,
> very especially whan the boards are small. Does anyone know, and is
willing
> to discuss, what work is therefore being done to improve ICT machines in
> terms of the following:
> a) using  the test solution at much higher starting resistivity (e.g.
>
>>200MOhm-cm +), assuming that at this level of resitivity, small amounts
of
>
> contamination will have a greater impact on the drop in resistivity when
> added (?)
> b) how the test solution is presented to the resistivity probe for
> measurement of these slight amounts of contamination. This is of especial
> concern since small amounts of contamination don't affect the resistivity
> of the test solution very much at current values (150MOhm-cm or so),
which
> thus remains relatively close to its starting value. If the test solution
> is not homgeneous and the measuring portion of the probe is not in
contact
> with all the test solution that passes it, solution of a higher or lower
> resistivity will not be measured and included in the machine's reading.
> c) the capability of the probes to accurately measure these relatively
very
> small differences in high resistvity values.
> d) the capability of the filters to clean the test solution without
adding
> further contanination back in.
>
> Suppose you have a board that only measures 4cm x 4cm, and it has to be
> 0.2ug NaCl eq/cm^2 clean. This means that the max amount of
salt-equivalent
> contamination allowable is only 6.4ug. To measure this board alone is
> impossible - no machine is remotely capable of accuratey  measuring a
> contamination level this low - so a batch of these little boards would
have
> to be tested all together and the reading taken across the combined board
> area. After being divided by the number of boards, you are left with an
> average reading only. This is maybe OK if you have a lot of boards to
test,
> but not so OK if you're repairing only one or two of them and want them
> clean again to original requirement.
>
> What is on the cards for future Ionic Contamination Testing equipment?
>
> Peter
>
> [This e-mail is confidential and may also be privileged. If you are not
the
> intended recipient, please delete it and notify us immediately; you
should
> not copy or use it for any purpose, nor disclose its contents to any
other
> person. Thank you.]
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>






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[This e-mail is confidential and may also be privileged. If you are not the
intended recipient, please delete it and notify us immediately; you should
not copy or use it for any purpose, nor disclose its contents to any other
person. Thank you.]

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